1. Field of the Invention
This invention relates to a reticle used for projecting a very fine pattern on a semiconductor substrate in photolithography and, more particularly, to a reticle into which phase-shifters are incorporated and to a method of fabricating the reticle with the phase-shifters.
With a trend toward higher integration of an integrated circuit, patterns formed on a semiconductor substrate in photolithography are required to be ever finer and more precise. A reticle having phase-shifters is one of the solutions therefor.
2. Description of the Related Art
It is known that a magnified pattern with a magnification ratio of about 1 to 10 is formed on a reticle and the reticle pattern is projected forming a reduced real image on a semiconductor substrate by a step and repeat method using a so-called stepper.
When dimensions constituting the pattern on the semiconductor substrate are reduced to submicron order, there arises a problem in that two lights which penetrate through two adjacent transparent portions of the reticle pattern, the two portions being separated by a narrow strip portion containing the pattern to be projected onto the substrate, interfere with each other, and accurate strip pattern is not projected and formed on the semiconductor substrate.
In order to solve the interference problem between the two adjacent lights, a phase-shifter technique has been introduced into the reticle fabrication process.
Typical known reticle structures with phase-shifters are described herein. FIG. 1 shows a cross section of a known reticle structure with phase-shifters. On a reticle substrate 1 manufacture of glass or quartz, a light-shielding layer 2 and 2' containing the pattern to be projected onto the substrate (hereinafter abbreviated as shield layer), generally a chromium (Cr) or a chromium compound is formed in a conventional method. Of the two adjacent openings 30 and 31, one opening 31 between shield layers 2 and 2' is covered with a phase-shift layer 3 (hereinafter abbreviated as phase-shifter) which has a function of improving the resolution in photolithography.
The lights 33 and 34 impinging on the bottom surface of the reticle and traveling a distance t, undergo a different phase shift due to an existence of the phase-shift layer 3 (in an actual case, the reticle is arranged upside down in the stepper). When a thickness of the phase-shift layer is properly selected, two lights passing through the reticle can be made to have an opposite phase (phase-shift of .pi.) to each other. This results in forming a distinct image pattern of the middle Cr layer 2' onto the semiconductor substrate because the adverse effects from the adjacent lights from both sides are eliminated.
FIG. 2 shows a cross section of another known reticle structure with phase-shifters. In the same way as in FIG. 1, a pattern of light-shielding layers 2 are formed on a glass substrate 1. However, in this case, a phase-shifter 3' has the shape of a groove or well formed under a window 31 between the two adjacent light-shielding layers 2 and 2'. The groove is formed by subjecting the substrate 1 to a conventional unisotropic etching process such as RIE (Reactive Ion Etching), in which only the substrate surface for the area where the groove is to be formed is exposed and the remaining areas are covered with a resist layer. The function of the phase-shifter 3' is almost the same as the phase-shifter 3 of FIG. 1. However, in this case, the light 33 traveling a distance d within the substrate causes a phase difference of .pi. from the light 34 which travels the same distance d through the groove without substrate material.
With a reticle with phase-shifters, fine complicated patterns are formed on a semiconductor substrate. However, in the process of fabricating a reticle, defects are often formed in the patterns of shield layer 2 or phase-shifters 3. In addition, though a reticle with phase-shifters has been completed, it may be required that a small portion of the pattern needs to be modified or changed. Therefore, it is an important that the reticle pattern be able to be repaired or modified.
When it is found that the shield layer 2 includes a missing portion or void portion, or an unnecessary extra portion in its pattern before forming phase-shifters, these defect portions are comparatively easily repaired using a Focused Ion Beam (hereinafter abbreviated as FIB) apparatus.
When the phase-shifter 3 is formed and includes a missing portion or an unnecessary extra portion in its pattern, the extra portion can be removed using the above FIB apparatus and the reticle can be repaired. However, if the defect is a missing portion in the phase-shifter pattern, the reticle can not be repaired easily.
The FIB apparatus will be briefly described. FIG. 3 shows a schematic cross section of the FIB apparatus. Ions are emitted from an ion gun 6 and gallium (Ga) ions are popularly used therein. The emitted ions are focused by a condenser lens 7 and further on-off controlled by a blanking electrode 8 and a blanking aperture 9. An object lens 10 and a deflection lens 11 have the function of focusing and projecting an ion beam onto an object 14 to be processed at a specified scanning position. The object can be moved by an XY-table 15. The ion beam can sputter-and-etch the object, and a secondary ion detector 12 can detect the object and identify the ion beam position. When a gas is introduced from a gas injection gun 13, the ion beam resolves or separates the gas, and the resolved elements or components can be deposited on the object 14.
In order to remove the unnecessary extra, portion of the shield layer 2 or the phase shifter 3, the unnecessary portion is subjected to irradiation of the above ion beam by moving the XY-table and scanning the ion beam. However, when the phase-shifter 3 of FIG. 1 has a missing portion, or when the groove-shaped phase-shifter 31 of FIG. 2 includes an extra groove portion which should not have been taken away, the process of depositing phase-shifter material or substrate material and repairing the void or missing portion of the phase-shifters is very difficult. The reason is that the thickness of phase-shifter 3 and depth of phase-shifter 3' is too large or deep (a few thousands angstroms) to bury or fill the void portion using the FIB apparatus. Even if silicon dioxide, which is a popular material for a phase-shifter, is deposited by the FIB apparatus, the deposited phase-shifter contains gallium. Since the gallium containing silicon dioxide is not transparent, silicon dioxide does not work as a phase-shifter.